Add ability to read /etc/passwd and /etc/group

Add the capability to read /etc/passwd and /etc/group for getpw* and
getgr* functions.

Bug: 27999086
Test: pwd, grp, grp_pwd_file unit tests
Test: Read in custom users/groups from /etc/{passwd,group}
Change-Id: Idc1f054af8a7ca34743a90493495f0ccc775a0d8
diff --git a/tests/Android.bp b/tests/Android.bp
index 25377df..1e5c13f 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -75,6 +75,7 @@
         "getcwd_test.cpp",
         "glob_test.cpp",
         "grp_pwd_test.cpp",
+        "grp_pwd_file_test.cpp",
         "iconv_test.cpp",
         "ifaddrs_test.cpp",
         "inttypes_test.cpp",